1. Field of the Invention
The present invention relates to a maximize efficiency method. More specifically, the present invention discloses a maximize efficiency method for resonant converter with self-adjusting switching points.
2. Description of the Prior Art
Power converters have transformers with primary and secondary windings in order to provide isolation. A switch such as a transistor is electrically coupled to the primary winding of the transformer. The switch controls the voltage transferring from the primary to the secondary winding. However, power loss occurs when the switch operates.
Refer to FIG. 1A, which is a schematic diagram illustrating a portion of a power converter circuit of the prior art and to FIG. 1B, which is a diagram illustrating voltages in the circuit of FIG. 1A.
The circuit 100 includes a transformer 110 having a primary winding PW and a secondary winding SW and a transistor 120 connected to the primary winding PW. An input voltage VIN is applied to the primary winding PW. A voltage VG is periodically applied (TON) to the gate of the transistor 120 to control the transfer of power from the primary winding PW to the secondary winding SW. When the transistor 120 is turned on, the energy is stored in the transformer 110. As the transistor 120 is turned off, the stored energy in the transformer 110 is discharged.
A reflected voltage VR is generated when the transistor 120 is turned off. As a result the voltage VDS across the transistor 120 equals the input voltage VIN plus the reflected voltage VR. While the transistor 120 is turned off, a parasitic capacitor inherent in the transistor 120 stores the energy from the voltage VD.
After a discharge period TDS the energy of the transformer 110 is fully discharge and the energy stored in the parasitic capacitor flows back to the input voltage VIN through the primary winding PW of the transformer 110.
The primary winding PW and the parasitic capacitor along with parasitic elements on the secondary winding create a resonant tank with a resonant frequency fR. While resonating, energy flows back and forth between the primary winding PW and the parasitic capacitor.
An ideal time to turn on the transistor 120 is when the lowest voltage level occurs after a delay time Tlow in order to reduce turn ON power loss to a minimum.
Therefore there is need for a more effective method of controlling the switching device in order to reduce switching power loss.